Cementing compositions including a dispersant agent for cementing operation in oil wells

ABSTRACT

The invention concerns a cementing composition for cold zones cementing operations in an oil well or analogous, said composition comprising a aqueous fluid base, solid particles including cement particles, and a dispersing agent in solution in the aqueous fluid base, said dispersing agent having a comb-like structure composed of a graft copolymer, said graft copolymer being constituted by a backbone carrying grafted chains.

The present invention relates to the technical field of cementing oil,gas, water, geothermal, or analogous wells. More precisely, theinvention relates to cementing compositions suitable for cementing incold zones.

The construction of an oil well is done by drilling subsequent sectionsuntil reaching the aimed reservoir zone. Each section is drilled whilecirculating a fluid which lubricates and transports the drill cuttingsout of the hole. After drilling each section a metal tube called acasing is lowered down the hole and a cement slurry is pumped throughthis casing to be placed into the annulus that extends between thecasing and the underground wall of the well.

The cementing aims at stabilizing the well and at isolating thedifferent subterranean zones so as to control the flow of fluids presentat different levels in the formations.

A cement slurry is a highly concentrated solution of solid particles.The aqueous base may be constituted by sea water or brine, and as aresult the slurry includes all sorts of ionic species. Additionally,cement properties are adjusted by the addition of various organic andinorganic additives including viscosifiers, retarders, accelerators,anti-settling agents and fluid loss control additives in order to adaptthe design of the cement slurry to the different parameters of thesection, e.g. temperature, type of formation and well geometry.

All of these species including the cement are subject to interactionsthat affect the flow and deformation of the slurry in response toapplied stresses, or in other words, the slurry rheology. Cementslurries are typical Non-Newtonian fluid, that is a flow threshold isoften observed, thus impeding displacement at low speed, and giving riseto sudden irregularities of flow. The apparent viscosity of the slurrymust be kept sufficiently low to avoid excessive friction pressures whenpumping the slurry into the annulus. Moreover, pumping a thick fluid inthe annulus bears the risk of exceeding the fracture pressure of theformation and would lead to strong losses of fluid. On the other hand,simply adding more liquid since the slurry density must balance that offluid formation cannot decrease the slurry viscosity.

Therefore, dispersants are added which reduce the fluid viscosity.Commonly used dispersants are polymers like polynaphtalenesulfonate(PNS) and polymelamine-sulfonate (PMS). These compounds are assumed toact by adsorbing onto the particles and disperse the slurry byelectrostatical repulsion. In general, these additives have only aslight retardation effect on the set time of the cement and this effectis negligible at the temperatures generally encountered in an oil well(above 50° C.). However, there is a range of operations that areperformed at lower temperatures (under 50° C.). This includes forexample cementing close to the sea water bottom or close to the surfacein deep water operations or permafrost areas.

In these cases accelerators are added to the cement slurry in order toachieve reasonable set times. Calcium chloride and sodium chloride areamong the most efficient and most economic accelerators and thereforeextensively used. However, addition of these substances to the slurrymake it more difficult to disperse. For instance, whereas for freshwater slurries about 0.5% to 1.5% of PMS (by weight of cement or BWOC)is typically required to achieve proper dispersion, concentration ashigh as 4% BWOC may be required for slurries containing sodium chloride.More generally, the required concentration may be from 5 to 10 timeshigher than the concentration required with fresh water when usingdispersants of the type mentioned above. This behavior is usuallyexplained by the conformational change of the charged polymer withincreasing ionic strength, i.e. screening of the anionic groups in thechain leading to contraction of the dispersant polymer.

A side effect of the increased concentration is an enhanced retardationeffect through the dispersant. This makes it necessary to add a higheramount of accelerator, which will demand a further addition ofdispersant and so on. Since the other admixtures and the cementparameters also effect the additive response, it is sometime impossibleto design a slurry that exhibits both a good rheology and an acceptablesetting time in cold environment.

A further drawback of these common dispersants is the strong increase ofthe slurry rheology that results from the cooling of the fluid. Thisproblem occurs for instance when pumping the slurry from a rig floorinto subterranean sections close to the sea bed: the slurry is preparedat sea level, at an ambient temperature for instance of 20 C and itstemperature decreases as it flows; an increase of its rheology may thuslead to excessive friction pressures. To obviate this problem, addinghigher amounts of dispersing agents is generally not suitable since itleads to over-dispersion and consequently to non stable slurries atmixing temperature Additionally, higher concentration would furtherlengthen the set time of the cement.

The present invention aims therefore at providing a new cementingcomposition suitable for oil well slurries that does not exhibit thedrawbacks of the compositions known of the art.

This aim is satisfied in the present invention a cementing compositionfor cold zones cementing operations in an oil well or analogous, saidcomposition comprising an aqueous fluid base, solid particles includingcement particles, and a dispersing agent in solution in the aqueousfluid base, said dispersing agent having a comb-like structure composedof a graft copolymer, said graft copolymer being constituted by abackbone carrying grafted chains.

In a preferred embodiment of the invention, the backbone is apolycarboxylic acid or a polyether chain and the grafted chains arecomposed of polymerised ethylenically unsaturated monomers. Preferably,said grafted chains are polyoxyethylene or polyoxyethylene/polypropylenechains.

The composition according to the invention is particularly advantageousbecause the structure of the dispersing agent shows no obviousretardation on the cement set time. Therefore, this composition is veryuseful to prepare slurries for zones down to 4° C. with very short settimes which can not be achieved with classical dispersants. Actually,the slurry rheology is independent of the temperature. Thus, itsfluidity remains constant when attaining colder zones as the rig floor,which avoids the increase of friction pressure in the annulus. Moreover,this composition has a very interesting economical aspect: in presenceof salt, the dispersant concentrations are up to 5-10 times lower thanwith common dispersants of the type PMS or PNS or similarpolyelectrolytes. Actually, when accelerators are required, thiscomposition makes the concentration necessary to obtain a well dispersedslurry independent of the said addition of accelerators. Finally, theuse of the comb-type dispersing agent does not lead to overdispersion ofthe slurry and can even help to reduce generation of free water.

Advantageously, the cementing composition further comprising anaccelerator agent, said accelerator being of the type of a chloridesalt.

The solid particles of the cementing composition of the invention maycomprise a trimodal blend, said blend being constituted by:

-   -   Portland cement having a mean particle diameter lying in the        range 20 microns (μm) to 50 μm;    -   microcement having a mean particle diameter lying in the range        0.5 μm to 5 μm; and    -   microspheres having a mean size lying in the range 20 μm to 350        μm.

This composition is of low density and is suitable to avoid exceedingthe fracture pressure of the formation, which could happen in operationsof cementation of sections in deep water wells. Actually, close to thesea bed, the formations are particularly weak and temperatures are below20° C. This composition further allows to have short set times attemperatures below 20-30° C.

Part or all of the cement particles may be constituted of microcementhaving an average particle diameter lying in the range of 4-5 microns.This composition, which cement has a higher surface area than a normalcement, is for example very interesting for squeezing casing leaks orfissured formations where a good fluid penetration is required.

In a preferred embodiment of the invention, the concentration of thedispersing agent is lying in the range of 0.5 g to 4 g of saiddispersant per kilogram of cement particles (including cement or cementand microcement).

The cementing compositions according to the invention present suchproperties that make them suitable for cementing operations in coldzones like zones close to the sea water bottom or zones close to thesurface in deep water operations or permafrost areas.

The following examples illustrate the invention without limiting itsscope.

In the following are presented several examples illustrating byexperimental data the numerous advantages of using a comb-typedispersant in an oil well slurry containing high amounts of salt.

Actually, it has now been found that the dispersion of a cement can becompletely disconnected from the accelerator concentration when usingdispersants with a comb like structure as described for example inpatent WO99/47468 or EP0271435. These dispersants are composed of abackbone, which can be for example polycarboxylic acid (like polyacrylicacid) or a polyether chain. The backbone carries grafted chains, whichare composed of polymerised ethylenically unsaturated monomers (forexample polyoxyethylene or polyoxyethylene/polypropylene side chains).

The structure of the polymer can differ in the length and type of thebackbone and the composition, number and length of the grafted chains.

For all tests a dispersant called Chrysofluid Premia100 (coded CP100Chryso, France) has been used to study the behaviour of comb-typedispersants, the concentration of this dispersant is lying in the rangeof 0.5 g to 4 g per kilogram of cement (meaning cement or cement andmicrocement). Some tests have been performed with other dispersants ofthe same type (e.g. Advaflow from Grace (US), Eucoflow346 from Fosroc)and gave nearly identical results (Table 1 bis). Thus, the results arerepresentative for the general behaviour of this type of dispersant(polycarboxylic polymers, esters with hydroxyl group-containingpolyethers).

EXAMPLE 1

In Table 1 are shown the data for a slurry dispersed with differentconcentrations of either the comb-type dispersant CP100 or PNS. Theslurry has been prepared with Gulf cement (a class G Portland cementsold by Dyckerhoff, Germany) at a density of 1.89 g/cm³ (15.8 ppg, poundper gallon) and contains 0.03 gallons per sack of anti foam agent (AFA)(i.e., 0.03 US gallons (3.78 litres) per 42 kilogram sack, 0.1 gps=9cm³/kg of cement).

TABLE 1 Slurry with CP100 Slurry with PNS Dispersant τ_(y) τ_(y) (cm³/kgof (lb_(f)/100 Viscosity Free Water (lb_(f)/100 Viscosity cement) ft²)(mPa · s) (ml) ft²) (mPa.s) Free Water (ml) 0 38.6 33.9 4 38.6 33.9 40.9 22.0 43.1 4 31.3 39.2 5 1.8 12.6 36.5 4 28.9 38.9 4 2.7 3.2 27.9 326.1 36.3 4.5 3.6 2.4 22.7 3 5.3 31.2 5.5 4.5 1.2 18.8 2.5 2.6 26.4 125.4 1.0 17.9 0 0.7 22.5 14.5

Dispersion with the latter leads to a strong drop of the yield stress(τ_(y), given in lbf/100 ft² i.e. libraforce per 100 square feet, theconversion in Pascals is given by multiplying by 0.478803) and theplastic viscosity (in milliPascals second or centiPoises) over arelatively narrow concentration range. Parallely with the dispersion alarge amount of free water is generated when slightly increasing the PNSconcentration. In contrary, with the comb-type dispersant slurryviscosity decreases quite gradually. This has the advantage that errorsin concentration do not lead to a drastic change in rheology. A furtheradvantage is that free water disappears when the cement slurry is fullydispersed.

EXAMPLE 1 BIS

In Table 1 bis are shown the data for a slurry dispersed with differentconcentrations of either the comb-type dispersants Advaflow from Grace(US) or Eucoflow 346 from Fosroc. The slurry has been prepared in thesame manner than in example 1.

Slurry A1 A2 A3 A4 PNS (cm³/kg slurry) 18 — — Eucoflow 346 (cm³/kgslurry) 1.8 Advaflow (cm³/kg slurry) — — 1.8 CP100 (cm³/kg slurry) 1.8AFA (cm³/kg slurry) 2.7 2.7 2.7 2.7 CaCl₂ (% BWOC) 2 2 2 2 Density(g/cm³) 1.89 1.89 1.89 1.89 Rheology 25 C. τ_(y) (lb_(f)/100 ft²) 2.6 77 2 Viscosity (mPa · s) 22 23 20 21 Thickening Time 10° C. 10° C. 12H305H45 7H45 7H30

These tests thus show that the result are approximately the same thanthe one on example 1 and are consequently representative of thesecomb-type dispersants.

EXAMPLE 2

Five slurry recipes are shown in Table 2 containing either PNS or acomb-type dispersant for control of the fluid rheology. An undispersedslurry (A5) is shown for comparison. For each dispersant is shown arecipe with and without Calcium chloride (acting as accelerator).

TABLE 2 Slurry A1 A2 A3 A4 A5 CP 100 (cm³/kg 2.25 2.25 — — — slurry) PNS(cm³/kg — —   3.6  27 — slurry) AFA (cm³/kg  2.7  2.7  2.7  2.7  2.7slurry) CaCl₂ (% BWOC)   0   2   0   2   2 Density (g/cm3) 1.89 1.891.89 1.89 1.89 Rheology 25° C. τ_(y) (lb_(f)/100 ft²)  0.5  0.6  5.3 1.2 26.3 Viscosity (mPa · s) 39.3 38.2 31.2 35.6 37.8 Rheology 10° C.τ_(y) (lb_(f)/100 ft²) —  0.2 —  3.3 — Viscosity (mPa · s) — 39.1 — 45.9— Thickening Time 25° C. 7H₂O  3H55 12H40  6H45  3H45 10° C. — 12H30 —18H15 — Compressive Strength Development (UCA) at 10° C. 3.45 MPa —17H10 — 26H 18H00 24 H — 7.6 MPa — 2.7 MPa 8.1 MPa

In absence of the accelerator 2 times more dispersant is needed in caseof PNS to achieve the same degree of dispersion as with CP100 (seeslurries A1 and A3). Addition of CaCl₂ to the slurry dispersed with CP10does not influence the rheology. In the slurry containing PNS, thedispersant concentration has to be raised by a factor 6 to achieve asimilar rheology as in slurry A3. When the temperature is lowered to 10°C. the rheology of the slurry A2 remains constant while the PNS in theslurry A4 looses some of its efficiency. The time required to attain acompressive strength of 500 psi (pounds force per square inch) (i.e.,3.45 MPa) is measured at 10° C. Further, the compressive strength after24 hours is measured (1 psi=6.894 kPascals). Values of 1100 psi (7.6MPa), 390 psi (2.7 MPa) and 1180 psi (8.1 MPa) have respectively beenmeasured for Slurries A2, A4, A5. Comparison of thickening time andcompressive strength development data of the dispersed slurriescontaining CaCl₂ (A2 and A4) with the undispersed slurry (A5) shows thatthe comb-type dispersant has no significant impact on the set propertiesof the slurry. PNS is retarding the cement set significantly.

EXAMPLE 3

Most low temperature applications are cementing operations relativelyclose to the surface (i.e. in general <1500 m). Lightweight slurries(i.e. with densities below 1.68 g/cm³-14 ppg) have to be used in generalto avoid exceeding the fracture pressure of the formation. One exampleis the cementation of sections in deep water wells. Close to the sea bedformations are very weak and temperatures can be below 20° C. Cementslurries are needed which have a low density and develop sufficientcompressive strength in a reasonable time at these low temperatures.

A usual extended lightweight slurry sets very slowly because of the highwater content. In Table 3 is shown a slurry design based on a trimodalconcept described in the Schlumberger French Patent FR-95 07010. Thistrimodal concept makes use of the difference in the size of solidparticles to improve the slurry and cement properties. This principleallows to design lightweight slurries with a much higher solid contentthan a normal extended slurry.

The trimodal blend consists of cement (Gulf from Dyckerhoff, class GPortland cement having particles with a mean diameter lying in the range20 microns (μm) to 50 μm), microcement (having a mean particle diameterlying in the range 0.5 μm to 5 μm) and microspheres (having a mean sizelying in the range 20 μm to 350 μm, and a specific gravity of 0.7g/cm³). The goal of the design is to provide a slurry with a low density(1.49 g/cm³-12.4 ppg) which shows short set times at temperatures below20-30° C. The good setting performance of the system is achieved by theaddition of the microcement (having a much higher surface area thannormal cement) and CaCl₂. The slurry further comprises an anti gazmigration agent (GMA), preferably considering those low temperatures afluid loss control agent comprising a micro-gel and a surfactantselected among the group consisting of polyvinylpyrrolidone, styrylphenol derivatives, N-alkyl pyrrolidones, with an alkyl chain of lessthan 12, alkoxylated alcohols, with an alkyl chain less or equal to 14and water soluble copolymers of vinyl pyrrolidone syuch as vinyl acetatewith a vinyl acetate content of less than 50%.

TABLE 3 Slurry B1 B2 Cement (% BVOB) 35 35 Microcement (% BVOB) 10 10Microspheres (% BVOB) 55 55 CP100 (cm³/kg blend) 3.6 — PNS (cm³/kgblend) — 27 AFA (cm³/kg blend) 2.7 2.7 CaCl₂ (% BWOB) 1 1 GMA (cm³/kgblend) 54 54 Density (g/cm³) 1.49 1.49 Rheology 25° C. τ_(y) (lb_(f)/100ft²) 7.0 17.2 Viscosity (mPa · s) 95.8 251.7 Rheology 10° C. τ_(y)(lb_(f)/100 ft²) 0.5 59.5 Viscosity (mPa · s) 164.3 507.9 Rheology 4° C.τ_(y) (lb_(f)/100 ft²) 0.5 n.m. Viscosity (mPa · s) 194.0 n.m.Thickening Time 25° C. 3H10  8H40 10° C. 4H30 13H15  4° C. 5H10 16H30Compressive Strength Development (UCA) at 25° C. 3.45 MPa 4H40 ? MPaafter 24 H 24.9 ? Mpa after 48 H 36.3 ? Compressive Strength Development(UCA) at 10° C. 3.4 Mpa 9H50 43H50 MPa after 24 H 15.2 — MPa after 48 H30.7 6.3 Compressive Strength Development (UCA) at 4° C. 3.4 Mpa 21H10 47H20 Mpa after 24 H 4.1 — MPa after 48 H 10.1 3.6 BVOB = By Volume OfBlend (sum of all solid particles including cement) BWOC = By Weight OfCement UCA = Ultra Sonic Analyser (normalised measurement mean)

As can be seen in Table 3 the amount of dispersant for achieving goodslurry dispersion is much higher in the case of PNS. In fact in slurryB2 a higher PNS concentration would be necessary to achieve a rheologyas low as in the design B1. However, further increasing the PNSconcentration led to a very strong retardation of the slurry leading tothickening times above 16 hours.

The results show that the comb-type dispersant allows to maintainpumpability when decreasing the temperature. The slurry B1 viscosifiessomewhat when decreasing the temperature from 25 down to 4° C. Thisviscosity increase can be related in a big part to the raise of theviscosity of water at lower temperatures. In contrary, a reduction oftemperature to 10° C. leads to a drastic thickening of the slurrydispersed with PNS. The rheology of the slurry B2 can be considered asbeing to thick for being pumpable in most applications. At 4° C. therheology was not measurable with the used rheometer (Fann35).

The slurry B1 shows no excessive increase of thickening time whenreducing the temperature from 25 to 4° C. A stronger impact of thetemperature reduction is observed for the cement set. The compressivestrengths after 24 hours and 48 hours have been measured. Thecompressive strength development becomes much slower, but 3.45 MPa (500psi) still can be reached in less than one day even at the lowesttemperature (500 psi is in general considered as the minimum strength atwhich drilling can be resumed). The compressive strength data for slurryB2 show the strong tendency of the PNS to retard the slurry. For thissystem about two days are needed to achieve a compressive strength of3.45 MPa (500 psi).

EXAMPLE 4

In Table 4 is presented a slurry prepared with a mixture ofapproximately 20% of Portland microcement and 80% of Slag (averageparticle diameter lying in the range of 4-5 microns). This cement has amuch higher surface area than a normal cement. Consequently, dispersionbecomes more difficult. This type of slurries is for example veryinteresting for squeezing casing leaks or fissured formations where agood fluid penetration is required. The system contains a polymericadditive (FLC) that improves the ability to penetrate into smallfissures (typically an additive that comprises a fluid loss controladditive of a nature and concentration such that the API fluid loss ofthe composition is less 30 ml/30 min).

TABLE 4 Microcement Slurry C1 C2 CP100 (cm³/kg cement) 9 — PNS (cm³/kgcement) — 18 AFA (cm³/kg cement) 4.5 4.5 CaCl₂ (% BWOC) 1 1 FLC (cm³/kgcement) 90 90 Density (g/cm³) 1.68 1.68 Rheology 25° C. τ_(y)(lb_(f)/100 ft²) 1.0 27.4 Viscosity (mPa · s) 75.0 109.3 Rheology 10° C.τ_(y) (lb_(f)/100 ft²) 0.5 60.5 Viscosity (mPa · s) 92.3 620.3Thickening Time 25° C. 5H20 14H30 10° C. 9H40 >24H    CompressiveStrength Development (UCA) at 25° C. 3.4 Mpa 9H22 22H10 Mpa after 24 H22.2 3.9 Mpa after 48 H 27.3 6.7 Compressive Strength Development (UCA)at 10° C. 3.14 MPa 9H50 n.a. MPa after 24 H 15.2 n.a. MPa after 48 H30.7 n.a.

The same observations are made as in examples 2 or 3. Dispersion withthe comb-type dispersant is much more efficient than with PNS. Therheology of slurry C1 remains relatively stable when decreasing thetemperature from 25 to 10° C. A strong thickening phenomenon is observedunder the same conditions for the slurry dispersed with PNS. Alsothickening time and compressive strength development stay in anacceptable range with the dispersant CP100. The set of the slurry ismuch slower in case of the slurry PNS. At 10° C. the thickening timeexceeds 24 hours and therefore the development of compressive strengthwas not analysed.

1. A well cementing method comprising pumping into a well in a cold zonea cementing composition comprising an aqueous fluid base, solidparticles including cement particles, and a dispersing agent in solutionin the aqueous fluid base, the dispersing agent having a comb-likestructure composed of a backbone, selected from the group consisting ofpolycarboxylic acid and polyether, carrying grafted chains ofpolymerized ethylenically unsaturated monomers, wherein said pumpingstep occurs at least partially through zones having a temperatureranging between 4° C. and 20° C.
 2. The method of claim 1, wherein thegrafted chains comprise polyoxyethylene chains.
 3. The method of claim1, wherein the grafted chains comprise polyoxyethylene/polypropylenechains.
 4. The method of claim 1, wherein the cementing compositionfurther comprises a chloride salt as an accelerator agent.
 5. The methodof claim 1, wherein the solid particles comprise a trimodal blendincluding: 30% to 40% of Portland cement having a mean particle diameterlying in the range 20 μm to 50 μm; 5% to 15% of microcement having amean particle diameter lying in the range 0.5 μm to 5 μm; and 50% to 60%of microspheres having a mean size lying in the range 20 μm to 350 μm.6. The method of claim 1, wherein the solid particles comprise a mixtureof microcement, having a mean particle diameter lying in the range of4-5 μm.
 7. The method of claim 1, wherein the cementing compositioncomprises 0.5 to 4 g of dispersing agent per kilogram of cement.
 8. Themethod of claim 1, wherein the well is located close to the sea bed. 9.The method of claim 1, wherein the well is located in deep water. 10.The method of claim 1, wherein the well is located in a permafrost area.